If the yarn is broken at a spinning station of an open-end rotor spinning machine, or when spinning is to commence at a spinning station, a trailing yarn end is brought back through the yam draw-off tube to the rotor in the respective spinning station, so that it can be combined therein with the fibers fed into the rotor. The spinning process starts with continuous feeding of fibers and drawing off the yarn. The insertion of the yarn into the rotor, as well as the feeding of the fibers, must be exactly matched to each other, so that the area of the pieced section does not differ in appearance or quality from the remaining yarn in an impermissible manner.
A piecing process requires particular care when feeding the fibers. If a yarn breaks, the sliver feed is immediately stopped. As a rule, the yarn is pulled out of the spinning chamber and therefore out of the rotor by means of the high winding speed. It is customary to clean the rotor prior to each piecing process before the yarn is returned into the spinning chamber. This measure is for the preventative purpose of increasing the quality of the yam and is intended to remove dirt which has collected in the rotor, as well as fibers which would hamper the piecing process. However, during this process the sliver being fed remains inserted in the opening device, and the opening roller continues to turn with the spinning vacuum being continuously applied to the spinning chamber. Thus, it cannot be ruled out that, after a cleaning process has been performed and while the stopped rotor awaits the piecing process, fibers are being transported in the direction toward the rotor and collect at the lowest point in the fiber collection trough of the rotor. These uncontrolled fibers flying around in the spinning chamber can form flocks in the yarn when being deposited on the rotor and can therefore trigger yarn faults. For this reason it is important that no stray fibers can get caught in the rotor prior to the piecing process. In order to be able to perform piecing and yarn formation without interfering fibers, it is known from German Patent Publication DE 44 45 740 A1 to generate an air flow in the spinning chamber by opening a flap, and to maintain this air flow until the speed (rpm) of the rotor has reached a value in which the centrifugal force of the fibers causes a deposition of the fibers in the rotor trough, which can no longer be affected by the airflow.
It is also known from German Published, Non-Examined Patent Application DE-OS 28 18 794 to influence the vacuum in the spinning chamber during the piecing process by opening a flap at an air supply opening of the spinning chamber, in order that, prior to feeding in fibers for piecing, the rotor is freed from fibers which have already been deposited therein, and so that the feeding speed of the fibers fed to the piecing device can be matched to the rotor rpm. Fibers and dirt, which adhere to the rim of the opening or have been caught when the flap was closed, can be aspirated from the vicinity through an opening unblocked by a flap. The deposition of dirt is made easier in particular by fibers carrying gummy or sticky substances. If the flap is prevented from closing completely, secondary airflows can occur in the spinning chamber, which have a negative effect on the feeding of the fibers to the rotor and thus on the spinning process.
It is further known from the earlier filed, but later published German Patent Publication DE 196 24 537 A1 to blow compressed air centrally into the rotor cup for the purpose of blowing out fibers interfering with the spinning process through an exhaust nozzle for the yarn located opposite the rotor. A valve which blocks a compressed air feed line is provided to this end in the cover over the spinning chamber, sometimes referred to as a hood. The valve consists of a sphere which is pressed against a sealing face by means of a spring acting counter to the spinning vacuum. The sphere is lifted off the sealing face by the effect of the compressed air flowing into the rotor cup, and the compressed air flows through the exhaust nozzle into the rotor. If fibers and dust on the sealing face prevent the complete seating of the sphere, secondary air can flow through the valve during the spinning process. If fibers and dust cling to the spring and in this way reduce its resiliency, a controlled air supply is no longer possible.